1. Field of the Invention
The present invention generally relates to a gain tilt compensator capable of compensating gain differences among wavelengths in wavelength division multiplexing (WDM) transmissions, and to a transmission system using the gain tilt compensator. More specifically, the present invention is directed to a control method of a variable type gain tilt compensator and to a transmission system using this variable type gain tilt compensator.
2. Description of the Related Art
In optical transmission systems, since optical signals are repeated/amplified by employing optical amplifiers without being converted into electric signals, cost-down aspects of long-distance transmission systems may be largely realized. As repeating/amplifying optical amplifiers of long-distance transmission systems, rare-earth-doped fiber amplifiers are mainly utilized at a present stage, while erbium-doped fiber amplifiers (simply abbreviated as “EDFA”) are typically known. For example, in such an EDFA, since either pumping light having a wavelength of approximately 1480 nm or pumping light having a wavelength of approximately 980 nm is entered into an erbium-doped fiber (simply abbreviated as “EDF”) into which erbium has been doped, optical signals having wavelengths defined from 1530 nm to 1620 nm can be amplified.
As a system capable of improving transmission capacity of an optical transmission system, the wavelength-division multiplexing (simply abbreviated as “WDM”) system has been proposed. The WDM system corresponds to such a system in which optical signals having many wavelengths are multiplexed, and then, the multiplexed optical signals are transmitted into a fiber. In this WDM system, since a wavelength multiplexing number is increased, a total transmission capacity is improved in proportion thereto. Since the above-described optical amplifier is capable of repeating/amplifying these WDM signals without demultiplexing these WDM signals, the cost reduction effect achieved by employing the optical amplifier can be furthermore increased.
In the case that a WDM signal is repeated/transmitted by optical amplifiers, a major factor for limiting a transmission distance is a wavelength dependent characteristic (namely, gain tilt) of a gain owned by this optical amplifier. When gains of optical amplifiers are made different from each other in response to signals (wavelengths), a difference between output power of signals (wavelengths), and a difference between signal-to-noise ratios (SNR) are conducted. As a result, a difference between characteristics of received signals is conducted. Also, a so-called “non-linear response” phenomenon is present in an optical fiber, by which waveform distortions and noise are increased, depending upon optical power (intensity) of incident signals. As a consequence, the output power difference between the signals, which is conducted by the gain tilt of the optical amplifier, may appear as a difference between influences caused by non-linear responses of this optical fiber, and furthermore, a difference between characteristics of the received signals is enlarged.
To suppress a gain tilt of an optical amplifier, a gain tilt compensator may be effectively applied. Such a gain tilt of the optical amplifier is mainly caused by a wavelength dependent characteristic of an EDF itself, or wavelength dependent characteristics of losses of optical amplifier components (except for EDF). Under such a circumstance, a loss characteristic of a gain tilt compensator is designed in order to cancel the wavelength dependent characteristics of these optical devices, and this gain tilt compensator is installed in the optical amplifier, so that the gain tilt may be suppressed. The gain tilt compensator may be realized by such an optical passive element as a fiber-Bragg grating (FBG), a dielectric multi-layer film filter, an etalon filter, or a Mach-Zehnder interferometer (MZ interferometer) which is formed on a glass waveguide.
Since the above-described gain tilt corresponds to a so-termed “static gain tilt”, namely the gain tilt amount does not depend upon a variation of either input signal power or pumping light power, the gain compensation can be performed by the above-explained fixed gain tilt compensator (namely, compensation amount is fixed). However, a dynamic gain tilt which is varied with respect to input signal power and/or pumping light power is contained in a gain tilt of an optical amplifier. There is a problem that this dynamic gain tilt cannot be compensated by the fixed gain tilt compensator.
FIG. 2 is a graphic diagram for representing a characteristic example of an amplifier output in the case that a 16-channel WDM signal is entered into an optical amplifier (EDFA). An automatic level control (simply abbreviated as “ALC”) is carried out in this optical amplifier, by which an pumping power amount is automatically adjusted in such a manner that even when input power of this optical fiber is varied, total output power becomes a constant value. Since the ALC control is performed, a summation of all signal power, namely total signal power is kept constant even when the input power is varied. However, when the input power is varied, the respective signal power will behave in such a way that this input power may seesaw, so that a signal power difference between wavelengths will occur. For instance, in the case that an input of an amplifier is −15 dBm, an output (relative value) of this amplifier is equal to 0 dBm, namely, a constant value irrespective of the wavelengths, and thus, a flat output characteristic is obtained. However, when the amplifier input is increased to become −10 dBm, since a signal output on the short wavelength side is decreased and a signal output on the long wavelength side is increased, an output characteristic will own a so-called “right-tilt-up” trend. To the contrary, when the amplifier input is decreased to become −20 dBm, since a signal output on the short wavelength side is increased and a signal output on the long wavelength side is decreased, an output characteristic will own a so-called “right-tilt-down” trend.
In an actual repeat/transmission system, input power of an amplifier is determined based upon both output power of the amplifier and a span loss between repeaters (loss of transmission path). The span loss is fluctuated every span due to such factors as a fluctuation in loss coefficients of a fiber itself, a splice loss, a connector loss when optical fibers are connected to each other, and a fluctuation in fiber lengths. More specifically, in a repeat/transmission system used on the lands, since it is practically difficult to correctly arrange repeaters in an equal interval, span loss contains fluctuations defined from several (dB) to 10 (dB), or higher.
In general, when an optical amplifier is manufactured, the compensation amount of the above-described fixed gain tilt compensator is designed in such a manner that a flat optical output may be realized with respect to a certain reference input value. However, when a fluctuation of span losses happens to occur, even if a flat output characteristic may be realized in the fixed gain tilt compensator as to a reference span loss corresponding to a reference input, this fixed gain tilt compensator cannot maintain this flat output characteristic due to an occurrence of a dynamic gain tilt in such a case that an input power variation caused by the fluctuation of the span losses is produced. Therefore, there is a problem in the repeating/transmitting operations of the optical signals.
For instance, such a case is considered in which an optical amplifier having the characteristic of FIG. 2 is employed as a repeater. As indicated in FIG. 2, it is so assumed that the reference input power by which flat output power may be obtained is defined as −15 dBm, and also, the reference span loss corresponding to this reference input power is determined as 20 dB. Assuming now that a span interval within a certain repeating section becomes shorter than a reference value, and a span loss becomes 15 dB, the amplifier input is increased to become −10 dBm. As a consequence, the amplifier output owns a so-called “right-tilt-up” trend. If the shortest wavelength is used as a reference, then a gain tilt of approximately 1 dB will occur. On the other hand, it is so assumed that since a span interval within a certain repeating section becomes longer than the reference value, or since excessively large loss is produced due to splice connections and connector connections, resulting span loss becomes 25 dB. As a result, the amplifier input is decreased to become −20 dBm, so that the amplifier output owns a so-called “right-tilt-down” trend. If the shortest wavelength is used as the reference, then a gain tilt of approximately −1 dB will occur.
In order to suppress such a dynamic gain tilt depending upon input power, the use of such a variable gain tilt compensator whose compensation amount is variable may constitute an effective means. As a means for realizing such a variable gain tilt compensator, for instance, in a publication 1, the variable gain tilt compensating device is realized by employing the Fraday rotator and the birefrigent device. Also, in a publication 2, the variable gain tilt compensating device is realized by the Mach-Zehnder interferometer formed on the glass waveguide. In both the variable gain tilt compensating devices, since the loss gradients (slope) with respect to the wavelengths are variably set, the dynamic gain tilts of the optical amplifiers depending upon the input power, as represented in FIG. 2, are compensated.
(Publication 1: N. Mitamura, H. Nagaeda, N. Shukunami, and N. Naganuma, N. Fukushima, “Flexibly Variable Spectrum Equalizer for Spectral Tilt Compensation”, Optical Fiber Communication Conference 2000, paper WF2)
(Publication 2: H. Hatayama, C. Hirose, K. Koyama, N. Akasaka and M. Nishimura, “Variable Attenuation Slope Compensator (VASC) Using Silica-based Planar Lightwave Circuit Technology for Active Gain Slope Control in EDFAs”, Optical Fiber Communication Conference 2000, paper WH7)
However, in order that these variable gain tilt compensators are actually operated in transmission systems, the below-mentioned control circuit is necessarily required. That is, this control circuit monitors amounts of occurring gain tilts, and determines gain equalizing amounts so as to set actual compensation amounts of these variable gain tilt compensators.
In particular, in order to monitor the gain tilt amounts, as represented in FIG. 3(a), the following operations are necessarily required. That is, a portion of output power is split by an optical power splitter 101, and a WDM spectrum is monitored by using a spectrum monitor 102 such as a spectrum analyzer, and then, the monitored result must be transferred to a control circuit 103. Generally speaking, the spectrum monitor 102 is very expensive. Also, in order to actually install this spectrum monitor 102 into a repeat/transmission apparatus and a terminal station, various aspects to be considered are still left in practical use in vies of space, durability, and reliability.
While utilizing such a fact that the dynamic gain tilt depending upon the input power owns the linearity with respect to the wavelength (shown in FIG. 2), a relatively simple tilt amount monitor indicated in FIG. 3(b) has been proposed in JP-A-10-22924, or JP-A-11-224967. In this tilt amount monitor, a portion of the output power is split by the optical power splitter 101, and this split output power is demultiplexed by a demultiplexer 104 into an optical signal “λ1” on the short wavelength and another optical signal “λ2” on the long wavelength, and then, these optical signals are converted into information “P1” and “P2” which are direct proportional to the signal power by a monitor photodiode 105 and another monitor photodiode 106, respectively. The control circuit 103 compares the signal power information P1 and P2 with each other. Assuming now that the optical signal “λ1” on the short wavelength side is defined as a wavelength on the shorter wavelength side than a center (in the vicinity of 1554 nm) of the dynamic gain tilt characteristic shown in FIG. 2, whereas the optical signal “λ2” is defined as a wavelength on the longer wavelength side than the center, both a sign of the gain tilt and an absolute amount of inclinations of the gain tilts may be calculated by comparing the signal power information P1 and P2 with each other.
However, in these conventional techniques, a large number of optical components such as the optical demultiplexer 104 and the monitor photodiode 106 are required, which many increase cost of optical repeaters. Also, when the total quantity of these optical components is increased, splice steps are increased, and mounting areas of these optical components are increased, and further, total cost is increased.